The present invention generally relates to a bearing unit, and more particularly, to a hydrodynamic bearing device in a thrust direction using a lubricating material, and to be employed for a main shaft portion of an apparatus rotating at high speeds such as a hard disc apparatus, polygon scanner apparatus or the like.
In recent years, there has been a trend to employ hydrodynamic bearings having a high rotational accuracy instead of a ball bearing, for a rotatory main shaft portion of a video head rotating assembly, hard disc apparatus, polygon scanner or the like, among which hydrodynamic bearings, particularly those using lubricating fluids such as oil, grease, etc., are adopted owing to the advantages that they have less troubles such as seizure and the like than in an aerodynamic bearing, and that they are high in the bearing stiffness in a wide range of revolutions from low speed to high speed.
Referring to FIGS. 6 to 8, one example of conventional hydrodynamic bearing devices in a thrust direction as employed for a hard disc apparatus will be described hereinafter.
In FIG. 6, the known hydrodynamic bearing device generally includes a base member 1, a shaft 2 fixed at its one end to the base member 1 by shrinkage fit or the like, a hub 3 having a bearing bore 3A for receiving the shaft 2 to constitute a radial bearing, and a thrust bearing member 4 mounted on the hub 3.
The shaft 2 is formed with herringbone grooves 2A and 2B, a flat upper end face 2C finished to be at right angles with respect to the shaft 2 and having a recess hole 2E for storing mixed foreign matters, and an annular groove 2D formed adjacent to the end portion of the shaft 2 as shown. The hub 3 has a space 3B formed at the upper portion of the bore 3A, and ventilation holes 3C and 3D.
As shown in FIG. 7, the thrust bearing member 4 is formed with spiral grooves 4A at its central portion, and a plurality of notches 4B at its peripheral portion for fixing by screws 13 or the like. In FIG. 6, a ring-shaped member 5 having helical grooves 5A in its inner periphery is incorporated within the space 3B together with a seal ring 6 and a C ring 9. Moreover, in the vicinity of the spiral grooves 4A (FIG. 7) of the thrust bearing member 4 and the helical grooves 5A of the ring-shaped member 5, a thrust lubricating material 7 composed of grease in a low viscosity or the like is contained. A radial lubricating material 8, for example of oil or the like is applied to the herringbone grooves 2A and 2B for the shaft 2. In FIG. 6, there are further shown a motor rotor 10, a motor stator 11, a grounding brush 14, and a disc 12 capable of magnetically or optically recording and reproducing signals.
Still referring to FIGS. 6 to 8, functions of the conventional hydrodynamic bearing device having the constructions as described so far will be explained hereinafter.
In the first place, when the motor stator 11 is energized, the motor rotor 10 starts rotation together with the hub 3, disc 12, thrust bearing member 4, ring-shaped member 5, seal ring 6 and C ring 9 for rotation without contact by the pumping action of the herringbone grooves 2A and 2B. Meanwhile, by the pumping action of the spiral groove 4A, the thrust bearing member 4 is brought into a floating or raised state for rotation without contact, and thus, the disc 12 effects the recording and reproduction of the signal by a head or the like (not shown).
In the above arrangement, the C ring 9 engages the annular groove 2D of the shaft 2 for retaining the rotating member or hub 3 in position, while the helical grooves 5A of the ring-shaped member 5 prevent flowing out of the lubricating material, by directing under pressure, the thrust lubricating material 7 poured in the vicinity of the spiral grooves 4A and helical grooves 5A, towards the side of the spiral grooves 4A by the pumping action of said helical grooves 5A. The seal ring 6 also functions to prevent the thrust lubricating material 7 from flowing out. The grounding brush 14 referred to earlier serves to prevent electrostatic destruction of the bearing oil film by causing the static electricity produced on the rotary member to flow towards the stationary member or base member 1.
However, the known arrangement as described so far has a serious disadvantage as follows.
As shown in FIG. 8, when the thrust bearing member 4 and ring-shaped member 5 rotate at high speeds (e.g. above 1,800 rpm), the helical grooves 5A tend to strongly agitate the thrust lubricating material 7 for scattering and flowing out as shown at 7A, 7B and 7C, thus giving rise to shortage of the lubricating material for the thrust bearing to reduce the amount of floating or rising of the rotary member or resulting in mixing of the scattered thrust lubricating material into the radial lubricating material, thereby deteriorating the performance as a radial lubricating material.